Doppler ultrasound fetal heart rate monitors insonate the fetal heart and surrounding tissue with high frequency sound. Echoes from internal tissues undergo Doppler shift proportional to the relative velocity of reflecting surface and transducer. The received ultrasound is demodulated to convert the Doppler signal to the audible range; it gives reassurance when the fetal heart can be heard in this way. Filters are used to reject signals from stationary and slowly moving tissue, and a processing algorithm is used to determine the time of occurrence of each heart beat and therefore the heart rate.
Such monitors suffer from conflicting requirements. For ease of use and versatility, the beam should be as wide as possible and penetrate to a great depth. However, for robust FHR detection, the sensitive region of the beam needs to be limited to a small volume around the fetal heart, rejecting echoes from other organs and moving tissue. Particularly problematic sources of unwanted echoes include fetal limbs, maternal blood vessels, the digestive tract, and in the case of multiple pregnancies, a sibling of the target fetus.
Furthermore, when the transducer moves slightly in relation to the mother's abdomen, typically when the mother changes position, large Doppler reflections are received from every point within the ultrasound beam. Such movement artefact is normally many times larger than the fetal signal and disrupts or confounds the extraction of fetal heart rate.
Some monitors use pulsed Doppler ultrasound which improves the signal-to-noise ratio (SNR) by gating the ultrasound receiver such that it only accepts signals within a certain range of times after the ultrasound pulse is transmitted. The opening and closing times of the gates are chosen to correspond to a desired transit time for the ultrasound and thus determine a maximum and minimum operating range for the ultrasound beam. Echoes from near tissues arrive too early to be detected while distant echoes arrive too late. Timing of the receive gate may be fixed or may vary under control of an algorithm in order to collect echoes from the locality of the fetal heart while rejecting unwanted echoes from other ranges.
Some systems offer a choice of ultrasound frequency. This is useful because attenuation of ultrasound in tissue is proportional to frequency, the sound from a lower frequency transducer penetrating to a greater depth than a higher frequency. Therefore, the user will select a low frequency when they require greatest range (for example with an overweight mother) but will select a higher frequency to avoid picking up unwanted echoes from deep organs or tissue in a slimmer mother.
Systems having fixed, wide receive gates may have difficulty extracting an accurate fetal heart rate when the signal contains a mixture of echoes from maternal blood vessels and fetal heart. This is especially problematic when the beam is not well-aimed at the fetal heart and the fetal signal is smaller or similar in amplitude to the maternal signal.
Systems with adaptive receive gate timing are able to narrow down the receive gate and track the fetal heart (at least in one dimension—the distance from the transducer) which gives them a better SNR than systems having a fixed, wide receive gate. This strength can also be a weakness however. By locking on to a signal source and ignoring signals from other depths, it is possible for the system to erroneously lock on the wrong signal; most commonly this would be a maternal blood vessel. For example, in a prior art system as shown in FIG. 1, the transducer (2) is incorrectly positioned on the maternal abdomen (1) such that the beam (3) does not insonate the fetal heart (6). The maternal descending aorta (4) is within the beam (3) and the system detects maternal heart rate because it is the only periodic signal available to it. Believing it has a valid fetal signal, the system narrows down it's receive gate until the sensitive volume is limited to the region (5). Even when the transducer is subsequently moved to the correct position as shown in FIG. 2, the system does not detect the fetal heart (6) although it is now in the beam (3) because the heart is not inside the sensitive region (5). This erroneous state could persist indefinitely.
The present invention aims to make improvements.